A. Field of the Invention
The present invention relates generally to the field of Radio Frequency Identification (hereinafter, “RFID”) systems, and more particularly to advanced RFID systems which employ battery supported RFID tags allowing a higher degree of active behavior in tags.
B. Background of the Invention
The applications and importance of RFID technology has significantly grown in recent years due to a number of reasons including improvements in IC processes, RFID standards development, government allocation of increased spectrum for RFID, and growing awareness of the value of automated tracking of assets. During this growth, RFID systems have progressed from relatively simple, lower-frequency systems to include more complex systems that operate in the longer-range Ultra-High Frequency spectrum. The lower-frequency, generally inductively-coupled systems are usually referred to as Low Frequency (hereinafter “LF”, approximately 100-150 KHz) and High Frequency (hereinafter “HF”, typically 13.56 MHz) systems. These system generally operate from ranges of a few centimeters to approximately 1 meter, and are limited in range due to the physics of “near field” communications that do not rely on a propagating electromagnetic wave. The systems operating in the Ultra-High Frequency (hereinafter “UHF”, typically 800 to 1000 MHz) range can have longer ranges due to more favorable physical propagation.
Passive LF, HF, and UHF RFID systems comprise tags that operate without batteries and effectively leverage power that is wirelessly received from an RFID reader to communicate information back to the reader. In the UHF case, this process is typically called “backscatter” and allows a passive tag to communicate with an RFID reader over limited distances. Because these tags are effectively powered by the field of an RFID reader, the distance the tag can communicate is limited by its own power consumption. As a result, passive UHF systems generally operate with practical ranges of several meters.
UHF passive backscatter RFID has enjoyed success in recent years due to the availability of small geometry integrated circuit processes that enable low cost integrated chips to go in the tags, low cost tag production and test processes, greater market awareness of RFID benefits, and effective standardization. In the standards arena, the EPCglobal™ Gen 2 standard has been particularly successful in growing the supply chain RFID market. The International Organization for Standardization (ISO) has also recently moved to adopt the EPCglobal™ Gen 2 version 1.2.0 passive standard and to extend it with higher performing battery assisted RFID under its ISO/IEC 18000-6C standard.
The EPCglobal™ Gen 2 standard is a “reader talks first” architecture where the reader “selects” categories of tags with a “Select” command. The chosen tags then are entered into an “interrogation round” or “query round” with a “Query” command. The Query command provides the tags with a random number size range indictor, and the tags generate a random number within that range that is then assigned to a counter to create a “time slot” for each tags. Tags seldom select the same time slot when the random number range is large compared to the number of tags within range of the reader. In the query round a “QueryRep” command then instructs each tag to decrement its random counter. When the tag count reaches zero, the tag “replies” with a 16 bit random number. If there are no collisions between tags that happened to have the same time slot, the reader replies with the same random number and the associated tag then transmits its unique identifying code. The process of separating tag replies in time effectively allows the tags to use low cost receivers that are on the same RF “channel” as far as the relatively broadband tags can tell. The process of the reader establishing contact with a particular tag is called “singulation”, and the process of the reader further interacting with the tag is called “access” In access, the reader can read and write data to tag memory.
To allow multiple readers to be in communications with overlapping populations of tags, EPCglobal™ Gen 2 based standards use the concept of “sessions”. A two bit variable called the “session” or “session code” identifies 4 sessions, each of which have a “Session” or “Inventory” flag with a “session state”. The flag is normally described as having state symbolic logic state “A” or “B”, where each of these can be mapped in a particular tag's internal design to either Boolean state “0” or “1”. Either “A” or “B” can be used to indicate that a tag has been successfully singulated or “inventoried”, though it is more common in practice to use “A” for “not yet inventoried” and “B” for “recently inventoried”. When up to four readers are simultaneously in range of a tag, these readers may each singulate the tag in near real time (the tag can be in the singulation process with all) via each reader using a different session, that is, using a different session code and the flag associated with that session.
Despite UHF RFID systems having extended range as compared to LF and HF, there are many applications needing a still longer operating range while also maintaining high reliability. Also, the limited range of the passive tags when the tags are in motion leads to limited time to conduct operations such as memory reads and writes. Sometimes even the time to send separate Select and Query commands for selective tag access is not reliably available.
Active RFID systems extend range by providing a power source and full featured radio on the tag. “Full featured” is intended to mean a highly sensitive and selective (interference rejecting) receiver and active transmitter whereby the tag creates its own transmit signal. These active systems can achieve ranges of hundreds of meters, but cost significantly more than passive systems. Additionally, the operational life of the active systems is limited by the batteries deployed within the tags and the ability to replace these batteries over the life of the system. Some applications, such as tracking of military supplies, can absorb the relatively higher cost of these active systems, but many others cannot.
To provide an intermediate level of performance between fully passive and fully active RFID systems, there has been over the last few years a movement to introduce “battery-assisted” or “semi-passive” RFID systems. These systems utilize the UHF band and extend upon passive tags by providing tag operating power from a compact battery such as a coin cell, thus enhancing range by eliminating the requirement for the tag to receive sufficient RF signal power to actually power itself from the signal. The tag may also utilize baseband signal gain to further enhance sensitivity. The tag maintains the use of a simple and low power “backscatter” transmitter that operates by modulating a reflection of a reader provided RF signal back to the reader. Standardization efforts have been underway within the International Standards Organization (ISO) to add semi-passive RFID technology to its EPCglobal™ Gen 2 based UHF RFID standard, ISO/IEC 18000-6C. The applicant is an active member of this organization and has contributed significantly to this particular effort.